1. Field of the Invention
The present invention relates to a process for the preparation of aldehydes and ketones
2. The Prior Art
Aldehydes and ketones are widely used in organic chemistry. For example, they are important precursors in the synthesis of heterocycles, or perfumes and dyestuffs.
A variety of processes are known for the preparation of aldehydes and ketones. For example, the formyl and the acyl group are successfully introduced directly into aromatic systems via electrophilic substitution reactions, which, however, are limited by the substitution rules. Aromatic ketones can also be synthesized via organometallic reactions. A disadvantage includes the necessity of using an anhydrous reaction medium. Another disadvantage is the use of toxic chemicals such as phosphorus oxychloride, carbon monoxide, zinc cyanide, mercury organyls and cadmium organyls.
Aldehydes and ketones may also be synthesized via oxidation reactions. An overview is given in the literature reviewed in Houben-Weyl, Vol. E3, p. 230 et seq., 1983, and Vol. 7/2a, p. 688 et seq. Customary oxidants are selenium dioxide, chromium trioxide, cerium(IV) ammonium nitrate in perchloric acid/nitric acid or manganese dioxide in concentrated sulfuric acid, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) or iodine in DMSO. These oxidants, however, are also not very suitable for larger-scale syntheses due to their toxicity, the high costs or difficult handling. Moreover, most syntheses are complicated and yield the desired aldehyde only in moderate yield. In accordance with Org. Synth. Coll. Vol. IV, 1961, p. 31, 4-aminobenzaldehyde can be synthesized from nitrotoluene with the aid of polysulfide. Purification of the reaction product, which has a tendency to polymerize, is difficult and must be carried out rapidly, so that this process is unsuitable for larger amounts of substance.
When carrying out the oxidation with oxygen with addition of catalysts, not only are the desired aldehydes formed, but in most cases also the corresponding carboxylic acids (see Houben-Weyl, Vol. E3, p. 234 et seq., 1983). If this process is carried out with addition of N-hydroxyphthalimide and Co(II) or Co(III) compounds, even the starting material is fully oxidized to give the carboxylic acid (Ishii et al., J. Org. Chem. 1996, 61, 4520). Aromatic ketones are formed from alkylbenzenes with the aid of N-hydroxyphthalimide and acetaldehyde/oxygen in nonaqueous medium (Einhorn et al. in Chem. Commun. 1997, 447). The formation of aromatic aldehydes from the corresponding methyl aromatics with the aid of the enzyme laccase and ABTS (2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) was described by Potthast in J. Org. Chem. 1995, 60, 4320. However, it was impossible to reproduce these results in independent experiments. Moreover, the laccase mentioned in the publication, by Mercian, which has an activity of 1.1.times.10.sup.4 (IU/ml, based on the conversion of 4-hydroxymandelic acid as substrate), is not available. When attempting to reproduce the results, an available product, namely Mercian laccase with an activity of approx. 95 IU/ml, was employed at 100-fold concentration. A difficulty is that such high amounts of enzyme would completely exclude an applicability of the method for preparative purposes. Under these conditions, not even traces of an oxidation of 4-nitrotoluene was observed. When 3,4-dimethoxytoluene was used in the conversion, only 0.3% of 3,4-dimethoxybenzaldehyde were detected. Synthetic methods using ABTS are generally limited by the price of the compound, which is high.
There is therefore a demand for an inexpensive process which allows even sensitive aldehydes to be synthesized on a large scale. In particular, there is a need for a process in which water can be used as reaction medium.